Re-etch apparatus with constant flow rate means

ABSTRACT

A mask etching apparatus has a succession of etching stations each including a spray means having a main valve to which etchant is supplied from a common source through a closed circulating distribution system including a supply manifold and a return manifold. Measuring devices monitor the etching process by measuring the aperture size of the mask in process and develop a control potential through which the etching time is controlled in each of the various stations. When no etching is to take place in a given station, its main valve is closed, simultaneously an auxiliary valve, which is connected in series with a fluid resistance element and is coupled between the supply manifold and return manifold, is opened. The resistance element has a resistance to fluid flow approximating that of the given etch station spray means and consequently the spray pressure at the other etching stations is maintained constant.

United States Patent [191 Lerner RE-ETCH APPARATUS WITH CONSTANT FLOW RATE MEANS I Martin L. Lerner, River Forest, 111.

Zenith Radio Corporation, Chicago, 111.

Filed: Feb. 2, 1973 Appl. No.: 328,980

Inventor:

Assi'gnee:

US. Cl. 156/345, 156/7 Int. Cl. C23f l/02 Field of Search 156/7, 8, 345, l6, 11;

References Cited UNITED STATES PATENTS 5/1972 Dietch 156/345 X 11/1972 Lerner 156/345 X Primary ExaminerWilliam A. Powell Attorney, Agent, or Firm-Nicholas A. Camasto; John J. Pederson June 11, 1974 5 7 ABSTRACT A mask etching apparatus has a succession of etching stations each including a spray means having a main valve to which etchant is supplied from a common source through a closed circulating distribution system including a supply manifold and a return manifold. Measuring devices monitor the etching process by measuring the aperture size of the mask in process and develop a control potential through which the etching time is controlled in each of the various stations.

When no etching is totake place in a given station, its main valve is closed, simultaneously an auxiliary valve, which is connected in series with a fluid resistance element and is coupled between the supply manifold and return manifold, is opened. The resistance element has a resistance to fluid flow approximating that of the given etch station spray means and consequently the spray pressure at the other etching stations is maintained constant.

3 Claims, 3 Drawing Figures Source 13 26 25 PATMEBJIII I I I874 FIG. 1

{'11 l De -pn k mm Y 5 3 a n db Mi PT. i 3m V S n m A n+ w 5 S 3 rm St m E 8 w T f r me O In y am H MM xo Q Q Etchont Source Densitometer Densitometer 3 Control Circuit ntrol Circuit Programmer RE-ETCH APPARATUS WITH CONSTANT FLOW RATE MEANS CROSS REFERENCES TO RELATED APPLICATIONS The subject application is a further development of the etching apparatus and process described in Us. Pat. No. 3,669,771 issued June 13, 1972, US. Pat. application Ser. No. 45,681 filed June 12, I970, and Us. Pat. No. 3,702,277 issued Nov. 7, 1972, all in the name of Martin Lerner and assigned to the assignee of the present invention.

BACKGROUND OF THE INVENTION ,The present invention is concerned with an improved apparatus and system for etching the shadow mask components of color cathode ray tubes. It is generally useful in providing such masks with a pattern of apertures of a predetermined size but is particularly advantageous in connection with re-etch systems for mask processing.

Re-etching is a technique that has been adopted in the manufacture of color picture tubes featuring blacksurround screens of the type described and claimed in US. Pat. No. 3,146,368, issued Aug. 24, 1964, in the names of Joseph P. Flore and Sam H. Kaplan. Such a screen in its preferred form is a mosaic structure comprised of a'multiplicity of phosphor dot triads disposed in an ordered array throughout the image or screen area. Each such traid includes adot of green phosphor, a dot of blue phosphor and a dot of red phosphor. The screen differs from conventional mosaic color screens by having phosphor dots reduced in dimension so that, instead of being in tangential contact with one another, they are separated from one another. Another particularly important and distinguishing feature is that the spaces intermediate the phosphor dots are covered with a light-absorbing material, such as graphite. In short, each phosphor dot has a circumscribing area of dark, light-absorbing material from which the screen derives its name black-surround."

It is common practice in screening such a tube to locate and dimension deposits of the various phosphor materials by photographic printing in which a photosensitive material is exposed to actinic energy directed from a source to the screen through the apertures of a shadow mask assigned to that screen and positioned in operating relation thereto during the exposure step. In order to have phosphor dots of reduced size, while retaining the efficiency and tolerance characteristic of conventional triad color tubes, it is distinctly preferred that the electron beams of the tube be larger in diameter than the phosphor dots. This obviously presents problems in screening, namely, the problem of establishing desired relative dimensions of phosphor dots to excitation electron beams.

A most successful process of screening, achieving the desired relative dimensions, features the use of a shadow mask having a pattern of holes initially dimensioned as required to screen the phosphor materials on the tube faceplate. After screening has been accomplished through well known photographic techniques, the mask is subjected to further etching to enlarge the holes to a precisely controlled size in order that electron beams, which obviously are dimensioned by the apertures of the mask through which the beams reach the screen, have a larger, and accurately determined,

diameter than the phosphor dots. This general process is referred to as etch-back" or re-etch. A control system for precisely dimensioning the mask apertures in the re-etch process is the subject of the aforesaid US. Pat. No. 3,669,771. US. Pat. application Ser. No. 45,681 discloses a preferred orientation of the mask in process, with particular regard to the location of the spray heads through which an etchant is directed to the mask during re-etch and US. Pat. No. 3,702,277 discloses a re-etch system including bypass valves for continuously recirculating etchant during those times when etching is not performed to thereby maintain the etch ant at a constant temperature.

It is, therefore, an object of the invention to improve the etch'back system for shadow mask processing.

It is another and particular object of the present invention to further improve the etching of apertures in the shadow mask to attain a predetermined and precisely controlled size.

SUMMARY OF THE INVENTION The invention provides in a system for etching the apertures of a color cathode ray tube shadow mask, wherein the system has a plurality of etch stations each having a spray means for directing an etchant vertically upward to contact and etch a mask, means for conveying the masks through each etch station seriatim with a swell time of a given duration in each such station, a fluid distribution system, including a source of fluid etchant, a supply manifold, and a return manifold for sustaining a flow of etchant from the source to the spray means and back to the source, a like plurality of main valves individually associated with an assigned one of the spray means, and a measuring system for measuring the aperture size of a mask after it has been etched in one of the stations to derive a control potential and for utilizing the control potential for selectively closing the main valveand controlling the etching of the mask in the next station, the improvement allowing more precise aperture size control than heretofore obtainable comprising a like plurality of fluid resistance elements each associated with an assigned one of the spray means, and each having a resistance to fluid flow approximating that of its assigned spray means. The improvement additionally comprises a like plurality of auxiliary valves, each associated with an assigned one of the main valves, and coupled in series with its corresponding fluid flow resistance element between the supply manifold and the return manifold and means coupling the measuring system to each auxiliary valve for selectively opening each auxiliary valve when a corresponding main valve is closed to thereby maintain a constant spray pressure at each etch station.

BRIEF DESCRIPTION OF THE DRAWINGS The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may be best understood by reference to the following description on connection with the accompanying drawings and in the several figures of which like reference numerals indicate identical elements and in which:

FIG. 1 represents an etching system embodying the invention;

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing, the system of FIG. 1 is for etching shadow masks or, as they are sometimes referred to, color-selection electrodes of the shadow mask type of color picture tubes. Although the system is useful when establishing a pattern of apertures in blank sheets of metal stock, it is of particular benefit in practicing re-etch and the screening of black-surround tubes and will be described in that connection simply as a matter of convenience.

The system is arranged for mass production in which a conveyor 8 presents a succession of shadow masks seriatim to the various work stages of the apparatus. The

first stage 10 to which the conveyor presents a mask is an oxide stripper. It is common practice to blacken the mask or provide it with an oxide coating so that its radiation properties are analogous to those of a black body. The oxide, however, resists the attack of an etchant that otherwise would be effective in enlarging the apertures of the mask in process. The mask usually is formed of cold rolled steel and a suitable etchant is ferric chloride. In order to avoid attenuating the effect of the etchant, the mask is treated at work station 10 with hydrochloric acid and an inhibitor which strip the oxide coating after which the mask is rinsed with water and conveyed to the following station.

The next stage 11 is a four-station etching arrange- .ment and each mask is advanced, step-bystep, through the various stations by means of conveyor 8 and provided with a dwell time of about 10 to 12 seconds per station. The dwell or process time available at each etching station is determined in the usual manner by means of a conventional programmer or timer 15 which controls the step-by-step advance of the conveyor as required to present each mask to the various etch stations and also energizes electrically actuated valves to control the etch process in properly timed relation to the travel of the shadow masks. It is convenient from the standpoint of uniformity and process control to have each of the etching stages fed from a common etchant source or supply 12 so that such parameters as concentration and temperature of the etchant are the same in each station. Source 12 and pump 16 are included in a closed fluid distribution system which sustains a flow of etchant between the source and the various etching stations as indicated by supply manifold 13 and return manifold 14. Also included in the distribution system and associated with each etch station are bypasses 26 which allow continuous recirculation of etchant even when its associated etch station is not etching to maintain a constant etchant temperature. Such bypasses are fully described and claimedin zles 21 distributed along a conduit 22 in communication with input manifold 13 so that the etchant passing through the spray head is directed in the form of an elongated narrow ribbon against the adjacent surface of mask 9. In order to distribute the etchant uniformly over the entire surface of the mask, the array of nozzles is rotatable as indicated by the pulley 23. A catch pan 24 or the like reclaims the etchant that falls back from the mask and delivers it to the return conduit 14.

As also shown in FIG. 1, an electrically actuated main valve 25 is provided for each station in series with its spray head and each such valve has associated with it an auxiliary valve 27 in series with a fluid resistance element 28, the series combination of the two being coupled between supply conduit 13 and return conduit 14. The resistance to fluid flow of fluid resistance element 28 approximates the resistance to etchant flow of its associated spray means 20. Fluid resistance element 28'may take the form of a pipe having an innerdiameter of adjustable dimension so that the resistance to fluid flow of element 28 can be adjusted to approximate the resistance to fluid flowof spray means 20. Fluid resistance element 28 is only effective during intervals when its associated main valve is closed.

FIG. 2 further illustrates the preferred posture for mask 9 in an etching station. As there indicated, the mask has a dome-shaped portion 9a which is provided with a pattern of apertures having an initial diameter as required to accomplish phosphor screening of the tube in fabrication. The aperture portion 9a is circumscribed by a flange-9b and the mask is positioned with that flange directed vertically upward. The flange has the customary leaf springs for removably affixing the mask in operative relation to its screen and these springs facilitate removably coupling the mask to the conveyer frame 30 provided with coasters for riding along rails 31 to advance the masks from one etch station to the next.

Each etching station may be equipped with'baffles that extend vertically downward from the ceiling of the station, leaving an opening sufficient for mask entry and exit or may have movable doors for opening and closing the entry and exit areas. In such a case, programmer 15 times or correlates the operation of the doors with the step movement of conveyor 8.

It has been determined empirically that there are random variations in the response of different shadow masks to the etchant in the re-etch process. Therefore, it is highly desirable to provide a system for measuring the aperture size of a mask after each stage of etching to derive a control potential for controlling etching in subsequent stages. This is accomplished by controlling the operation of main valve 25 at each station to determine the amount of etchingif any, that is to occur in those stations. A densitometer control 35, as described in U.S. Pat. No. 3,669,771 is associated with each etching station. As indicated in FIG. 2 it comprises a light source 36 and a photocell 37 positioned on opposite sides of the path of travel of the mask and determines the aperture size thereof by measuringthe masks transmissivity to light. Obviously, large apertures in the mask will permit more of the light emanating from source 36 to reach photocell 37. Photocell 37 is coupled to a densitometer control circuit 38 which essentially is a voltage comparison device and develops, as an output, a control voltage. The densitometer is calibrated to develop an appropriate voltage for determindensitometer 35 arranged so that the densitometer of the first, second and third stages control the duration of etching in the second, third and fourth stages, re-' spectively. A densitometer may be used at the fourth station (as shown) to provide an indication or measure of the final aperture size of the mask. The output voltage of the densitometer of one station controls energization of the electrically operated main valve 25 of the next station. The system timing cycle is preferably adjusted to require etching in more than one station.

a color cathode ray tube which feature blacksurround.

While a particular embodiment of the invention has been shown and described, modifications may be 5 made, and it is intended in the appended claims to cover all such modifications as may fall within the true spirit and scope of the invention.

I claim:

1. A re-etch apparatus for etching the apertures of a 10 color cathode ray tube shadow mask to a precisely con- Thus, after the first etching cycle, the control voltage 1 developed by the first densitometer will permit main valve of the second etch station to remain open for an appropriate portion of the second etch cycle, when the mask will be in the second etching station. If the densitometer associated with the second station determines that the desired final aperture size has been attained during the second etch cycle, its content voltage will close main valve 25 of the third station during the third etch cycle and prevent etching in the third station. The same result is occasioned at the fourth station because the third station densitometer will also indicate appropriate final aperture size, all in accordance with the complete description in Lerner U.S. Pat. No. 3,669,77 l.

During any operating interval or etching cycle in which one or more valves 25 is closed, the pressure of manifold 13 will increase because of the decrease fluid flow in the system. As well known, an increased etching rate is obtained when the etchant spray pressure is increased. For optimumprocess control, the spray pressure at each etching station should remain constant. Constant spray pressure is obtained in the described system with auxiliary valves 27 which are opened or closed by densitometers or 38 in reciprocal relation to the opening and closing of their associated main valves 25. The etchant which would have poured through the main valve of a given station is directed through the auxiliary valve and fluid resistance element. Since the resistance to etchant flow of fluid resistance elements 28 is of its associated spray means 20, the system fluid flow remains the same and the pressure at manifold 13 doesnt change. Consequently, the spray pressure at the etching stations is not affected by closing of one or more main valves 25. Of course, when an etch station is to etch a mask, programmer 15 opens its main valve 25 and closes its auxiliary valve 27. Therefore, the absence or presence of actual etching in one or more of the etching stations does not affect the spray pressure at any of the other etching stations.

Fluid resistance element 28 as shown in FIG. 3 may be an orifice plate 40 interposed between two connecting pipe flanges, 41 and 42, the size of the orifice being adjusted to provide the desired resistance to etchant flow. Alternatively, element 28 may be a screen device, a partially opened valve, or any other arrangement for impeding fluid flow.v

The described arrangement has been found to improve the re-etch process. In particular, it minimizes variations in spray pressure at the various etching sta- 6 tions thereby permitting more precise control of hole size, whichis highly desirable. This is of particular importance for optimizing the operating characteristics of,

trolled degree comprising:

a source of fluid etchant; Y

a plurality of etching stations, each said station in cluding a spray means for spraying said fluid etchant onto said mask;

a closed distribution system including a supply manifold and a return manifold for supplying said etch ant to said spray means and for returning said etchant to said source;

a like plurality of main valves, each associated with an assigned one of said spray means for interrupting the flow of said etchant to said spray means;

a like plurality of auxiliary valves each associated with an assigned one of said spray. means;

a like plurality of fluid resistance elements, each associated with an assigned one of said spray means,

each having a resistance to fluid flow approximating that of its assigned spray means, and each coupled in series combination with its auxiliary valve between said supply manifold and said return manifold; and

means coupled to said main valves and to said auxiliary valves for opening and closing each said auxiliary valve and its assigned main valve in reciprocal relation to thereby maintain a constant spray pressure at each said spray means regardless of the opened or closed condition of said main valves.

2. In a system for etching the apertures of a color cathode ray tube shadow mask said system having a plurality of etching stations each having a spray means for directing an etchant vertically upward to contact and etch a mask, means for conveying the masks through each etching station seriatim with a dwell time of a given duration in each such station, a fluid distribution system, including a source of fluid etchant, a supply manifold, and a return manifold for sustaining a flow of etchant from said source to said spray means and back to said source, a like plurality of main valves individually associated with an assigned one of said spray means, and a measuring system for measuring the aperature size of a mask after it has been etched in one of said stations to derive a control potential and for utilizing said control potential for selectively closing said main valves and controlling the etching of said mask in subsequent stations, the improvement allowing more precise aperture size control than heretofore. obtainable which comprises:

a like plurality of fluid resistance elements each associated with an assigned one of said spray means, and each having a resistance to fluid flow approximating that of its assigned spray means;

a like plurality of auxiliary valves, each associated with an assigned one of said main valves, and coupled in series with its corresponding fluid flow resistance element between said supply manifold and said return manifold; and

3 ,8 l 6,2 l 4 a 7 a 8 means coupling said measuring system to each said sure at each said etch station.

auxiliary valve for selectively opening each said 3. A re-etch apparatus'in accordance with claim 2 auxiliary valve when a corresponding main valve is where-said fluid resistance element is an orifice plate. closed to thereby maintain a constant spray pres- 

1. A re-etch apparatus for etching the apertures of a color cathode ray tube shadow mask to a precisely controlled degree comprising: a source of fluid etchant; a plurality of etching stations, each said station including a spray means for spraying said fluid etchant onto said mask; a closed distribution system including a supply manifold and a return manifold for supplying said etchant to said spray means and for returning said etchant to said source; a like plurality of main valves, each associated with an assigned one of said spray means for interrupting the flow of said etchant to said spray means; a like plurality of auxiliary valves each associated with an assigned one of said spray means; a like plurality of fluid resistance elements, each associated with an assigned one of said spray means, each having a resistance to fluid flow approximating that of its assigned spray means, and each coupled in series combination with its auxiliary valve between said supply manifold and said return manifold; and means coupled to said main valves and to said auxiliary valves for opening and closing each said auxiliary valve and its assigned main valve in reciprocal relation to thereby Maintain a constant spray pressure at each said spray means regardless of the opened or closed condition of said main valves.
 2. In a system for etching the apertures of a color cathode ray tube shadow mask said system having a plurality of etching stations each having a spray means for directing an etchant vertically upward to contact and etch a mask, means for conveying the masks through each etching station seriatim with a dwell time of a given duration in each such station, a fluid distribution system, including a source of fluid etchant, a supply manifold, and a return manifold for sustaining a flow of etchant from said source to said spray means and back to said source, a like plurality of main valves individually associated with an assigned one of said spray means, and a measuring system for measuring the aperature size of a mask after it has been etched in one of said stations to derive a control potential and for utilizing said control potential for selectively closing said main valves and controlling the etching of said mask in subsequent stations, the improvement allowing more precise aperture size control than heretofore obtainable which comprises: a like plurality of fluid resistance elements each associated with an assigned one of said spray means, and each having a resistance to fluid flow approximating that of its assigned spray means; a like plurality of auxiliary valves, each associated with an assigned one of said main valves, and coupled in series with its corresponding fluid flow resistance element between said supply manifold and said return manifold; and means coupling said measuring system to each said auxiliary valve for selectively opening each said auxiliary valve when a corresponding main valve is closed to thereby maintain a constant spray pressure at each said etch station.
 3. A re-etch apparatus in accordance with claim 2 where said fluid resistance element is an orifice plate. 